a. Field of the Invention
The present invention pertains generally to the fusion of plastic pipe and specifically to the fusion of plastic pipe wherein seams created by the fusion process are minimized.
b. Description of the Background
Polyethylene pipe is used commercially for the construction of pipelines for various applications such as natural gas, water, sewer, and other materials. In addition, polyethylene pipe may be used as conduit to protect transmission cables such as high voltage electricity, fiber optic telecommunication, telephone, cable television, and other signal cables.
Polyethylene pipe is typically joined through a process called fusion welding, the most common joint being a butt joint. A butt joint is formed by holding the two sections of pipe rigidly, performing a facing operation to square the ends of the pipes and prepare the pipes for welding, then heating the prepared faces of the pipes on a hot plate to the melting point, removing the heating element, and forcing the two melted faces together. The resulting joint may be cooled before being subjected to any handling forces. The result is a fused joint that is as strong or stronger than the parent material and is very reliable.
U.S. Pat. No. 4,990,209 issued to George Rakes entitled “Self Propelled Fusion Machine” (Rakes) is a machine adapted to perform butt joints on polyethylene pipe as described above. Rakes is specifically incorporated herein by reference for all that it discloses and teaches. The Rakes machine performs the sequence of butt fusing in a semi automated fashion, with provisions for easily loading and unloading the pipe as it is welded. Additionally, the Rakes device performs the butt fusion process in an enclosed environment, adding to the consistency and throughput of the machine in all types of weather.
FIG. 1 illustrates cross section of a butt joint made on the Rakes machine or any similar machine common in the industry. A first pipe 102 and a second pipe 104 are joined at the weld line 106. During the fusion process, the material in the zone 108 is melted and then forced together. When the pipes 102 and 104 are forced together, excess melted material forms outer lips 110 and 112 and inner lips 114 and 116. The material along the weld line 106 solidly and rigidly joins together, forming a very strong and reliable joint.
The inner lips 114 and 116 can create problems in certain applications. For example, when a pipeline is used as a conduit, the maximum amount of wires or transmission cables that may be installed in the conduit are limited by the innermost diameter of the lips 114 and 116. In such cases, the pipeline constructor may be forced to use a larger diameter pipe to compensate for the effects of the lips 114 and 116.
The inner lips 114 and 116 may create problems when the pipe is used as a conduit. The lips 114 and 116 may catch or impede the cables or transmission lines when the lines are installed into the conduit after it is buried. In such cases, it would be advantageous to remove or minimize the lips 114 and 116.
The inner lips 114 and 116 may be cut away by inserting a cutter inside the pipe. Such an operation is time consuming and costly. Not only is it difficult to insert a cutter deep into the pipe, but the chips from the cutting operation may be difficult to remove completely.
In some cases, the inner lips 114 and 116 may impede the function of the pipe. For example, in the case of a sewer pipe wherein semi solid and other wastes in a water stream flow by gravity, the lips 114 and 116 may cause some of the wastes to collect and be prevented from flowing to their intended destination.
In some cases, the outer lips 110 and 112 may impede the function of the pipe. For example, when the assembled pipe is installed using a feeder system that moves and guides the pipe into position, the lips 110 and 112 around the outside of the pipe may catch on the feeder apparatus. In such cases, the lips 110 and 112 may be removed manually or with a machine as a secondary operation.
It would therefore be advantageous to provide a method of fusing pipe wherein the effects of lips commonly formed during the fusion process are minimized or eliminated. It would be further advantageous for the process of eliminating the effects of the lips to be simple, easy to implement, and consistent from weld to weld.